Carbon dioxide is a well-known gas, which is present in the atmosphere. It is released to the atmosphere in large amounts by fermentation processes, limestone calcinations, and all forms of combustion processes of carbon and carbon compounds. In the recent decades, the attention in respect of said emission has been rising, because of the environmental problem due to future climate change via Greenhouse effect. Consequently, extensive work has been performed over the years in order to develop processes for the removal of carbon dioxide from combustion gases. If possible, a subsequent recovery of carbon dioxide may make those processes economical feasible.
One type of conventional methods for the recovery of carbon dioxide from a gaseous source is the absorption method, in which carbon dioxide is absorbed in an absorbing agent. If other gases, such as oxygen, are present in the gaseous source, said other gases may also be absorbed chemically and/or physically. This will be the case if an amine-based agent is used as the absorbing agent.
It is well-known from the prior art that when O2 is present in the carbon dioxide-containing gaseous source and when alkanolamine is used as the absorbing agent, said O2 will be transferred into the alkanolamine-containing absorbing agent during the absorption procedure. As a consequence an unwanted degradation of alkanolamine as well as corrosion problems will occur due to the presence of O2.
Many prior art documents relate to this problem. EP 1 059 110 discloses a system for recovering absorbate such as carbon dioxide using an alkanolamine absorbent fluid, wherein the loaded absorbent is heated in a two step heating procedure prior to the separation of the absorbate from the absorbent, and wherein the loaded absorbent is deoxygenated after the first heating step and prior to the second heating step. The deoxygenation takes place by means of depressurisation.
In EP 1 061 045 a system for recovering absorbate such as carbon dioxide from an oxygen-containing mixture is described, wherein carbon dioxide is concentrated in an alkanolamine-containing absorption fluid, oxygen is separated from the absorption fluid, and carbon dioxide is steam stripped from the absorption fluid and recovered. In this system, the oxygen is separated from the absorption fluid by passing the carbon dioxide loaded absorbent comprising dissolved oxygen in countercurrent mass transfer contact with oxygen scavenging gas.
In other cases nitrogen oxides (also named NOx), sulphurous compounds and volatile organics may be present as contaminants in addition to O2 in the gaseous source. These contaminants will also be absorbed chemically and/or physically in the absorbing agent, when an amine-based agent is used as the absorbing agent.
In a conventional plant for production of high purity carbon dioxide, the carbon dioxide is firstly absorbed in an absorbing agent and afterwards the carbon dioxide and the absorbing agent is separated in a stripper column. However, part of the contaminants present in the feed gas is absorbed together with carbon dioxide during the absorption step. When separating the carbon dioxide from the absorbing agent in a subsequent stripper process, part of the absorbed contaminants will also be released in the stripper off gas together with the carbon dioxide. The stripper off gas will further contain N2 and O2 in some amounts.
When producing food grade carbon dioxide or other carbon dioxide applications, where a high purity is required, these contaminants must be removed from the stripper off gas in down stream equipment in order to obtain the required purity. Conventional technology available for removing such contaminants includes scrubbing, oxidation, adsorption and distillation.
The first step of the down stream purification of the stripper off gas is most often an oxidation process. In this oxidation step any NOx's present is oxidised to nitrate, which subsequently may be removed as a liquid phase. Furthermore, if sulphur is present as hydrogen sulphide, this compound is oxidised to free sulphur. Unfortunately, this oxidation requires a large demand of chemicals. Various oxidation agents may be used. In particular, potassium permanganate is widely used. However, this particular chemical is highly hazardous and, furthermore, as potassium permanganate may be used for the production of explosives it is to be expected that commercial use of this chemical at some point may be forbidden.
In the next step of the down stream purification the carbon dioxide containing gas is passed to a dehydrator. In this dehydrator any water present in the gas is absorbed and thereby removed from the gas stream. However, if any residues of acetaldehyde and/or volatile oxygenates are present in the gas these compounds are also removed in the dehydrator.
In the last step of the down stream purification the gaseous carbon dioxide is liquefied in a condenser. In the condenser it is possible to remove any residues of NO, which may still be present. This, however, is not the case for any residues of NO2. In fact if any NO2 is present when the gas reaches the condenser, or if any NO2 is produced inside of the condenser as for example due to oxidation of NO, said NO2 will be transferred to the liquid phase in the condenser and is subsequently almost impossible to remove.
Hence, an object of the present invention is to provide a method for the recovery of high purity carbon dioxide, which is substantially free of contaminants, wherein the oxidation step in the down stream purification is not required.
In the presently pending international patent application PCT/DK2006/000417 the present inventors found that by introducing a flash column between the absorption column and the stripper column the content of NOx in the stripper off gas can be markedly reduced, when an alkanolamine is used as the absorption agent.
Surprisingly, it has now been found that in addition to NOx's also sulphurous compounds and volatile organic contaminants can be efficiently removed in the flash column irrespective of the type of absorption agent chosen for the performance of the absorption of carbon dioxide in the absorption column.
Hence, by introducing this flash column several beneficial effects are obtained.
First of all, the content of contaminants in the stripper off gas is so low that the subsequent oxidation is no longer required. Hence, the consumption of chemicals is reduced and no subsequent disposal of these used chemicals is necessary.
Secondly, substantially no NO2 is present in the liquid carbon dioxide phase leaving the condenser. This is due to the fact that as almost all O2 has been removed in the flash column, the chemical equilibrium NO+½O2<->NO2, will shift to the left to form mainly NO. Therefore, no NO2 is present in the gas, when the gas later on enters the condenser. Furthermore, no NO2 is produced in the condenser because of the very low content of O2 in the gas. Hence, substantially no NO2 can be found in the liquid carbon dioxide product.
Thirdly, it has been found that the energy consumption for operating the condenser is markedly reduced. This effect is considered to be due to the fact that the very low content of contaminants in the gas phase will increase the overall heat transfer coefficient as well as the dew point temperature compared to conventional plants. This leads to higher suction pressure for the refrigeration compressors for the condensation process, which also leads to a reduced energy requirement for the condensation.
Fourthly, the product yield is increased because the loss of gaseous carbon dioxide in the condenser is markedly lowered as compared to conventional plants for production of high purity carbon dioxide.